Normal pregnancy blood pressure
Your blood pressure is taken at every pregnancy visit as well as intermittently during labour and occasionally after the birth, depending on your health. When a caregiver takes a person's blood pressure, they measure the pressure exerted by the blood against the walls of the blood vessels in the arteries.. The reading is determined by many things including:
The pumping action of the heart,
or how hard the heart pumps the blood. This is the highest reading (eg. 120 mmHg if your blood pressure is 120/80) and is known as the systolic measurement, which fluctuates with stress, emotion, anxiety and exercise, possibly increasing this reading.
The lower reading is the pressure on the blood vessels between the heart pumps or when the heart is at rest (eg. 80mmHg if your blood pressure is 120/80), known as the diastolic measurement. The diastolic pressure is the constant, lower pressure on the blood vessels and generally the measurement your caregiver focuses on during pregnancy.
Resistance to the flow of blood in the smaller arteries (or arterioles).
The smaller arteries in the body can control blood pressure through tightening and relaxing. If they tighten, they increase their resistance and do not allow blood to flow as freely, increasing blood pressure and decreasing blood flow to the smaller vessels in the body (as well as possibly the placenta and baby during pregnancy). This is one physical side effect of very high blood pressure during pregnancy called pre-eclampsia. A tightening of the arterioles is also caused by smoking. You can read more in smoking and lifestyle changes during pregnancy.
Elasticity of the walls of the large arteries.